In vacuum cleaners utilizing suction to pick up dirt and dust, it is known that the air flow through the vacuum cleaner, after having the dust and dirt particles removed, functions to cool the motor. However, this air flow is reduced successively upon continued use of the vacuum cleaner due to the clogging of the pores in the container walls, and the increased pressure reduction by the container walls. The result is an increased load on the motor as well as the risk of overheating of the motor.
It is known to provide overheating protection to a vacuum cleaner motor in order to avoid damage thereto due to insufficient cooling of the motor when the dust cannister is clogged to a certain degree. One such overheating protection device is described in Swedish Pat. No. 192,337 in which a temperature sensing body is used that is filled with liquid and connected to a servomotor constituted of a closed bellows. In this device, when the motor temperature increases, the liquid present in the device expands and the volume increase is propelled through a conduit to the bellows which then enlarges lengthwise. The bellows is caused to move and this movement is transmitted through a rod to a valve body that moves to open an opening in the casing for supplying atmospheric air to the motor-fan unit. Thus, air, in this known arrangement, moves directly to the fan without first passing through the dust container. Desirably, the motor is cooled to a normal operating temperature, and the risk of overloading and overheating the motor is eliminated.
The above described known device has a serious drawback in that it is expensive to manufacture because of its relatively complex construction. Furthermore, the use of liquid-filled receptacles and conduits presents a danger in that there is always a risk of leakage from this arrangement due to the vibrations which occur when the vacuum cleaner motor is switched on. Interruption of the vacuum cleaner operation may be the unwanted result.